Today, LED-based lighting units are being used for many applications. The low power consumption and long lifetime of LEDs make them a very useful alternative to conventional light sources like incandescent lamps or light tubes. Hence, not only new-designed lighting equipment often utilizes LEDs, but in many markets LED products are being used to replace other light-sources like incandescent or halogen light sources, for instance. These so-called retrofit products have to be compatible with existing lighting/power supply systems.
In many lighting system topologies, dimmed light is needed. In the latter case, a phase-cut power supply/dimmer typically is placed between the lamp and mains. Here, the time evolution of the resulting voltage is a phase-cut sine wave (created by the dimmer).
Two types of phase-cut dimmers can be applied: trailing-edge dimmers and leading-edge dimmers. In both types of dimmers, a part of sinusoidal mains voltage is cut out either from the front part (leading-edge dimmer) or from the end part (trailing-edge dimmer) of the sine half-cycle to reduce the power flowing to the lamp load. Depending on the desired degree of dimming, the timing of the phase-cut edge may be adjusted so that a smaller or larger part of the mains voltage is cut out.
Trailing-edge dimmers are usually MOSFET-based and comprise an internal supply circuit which powers the timing and zero-crossing detection circuit. Leading-edge dimmers typically are TRIAC-based or based on two anti-parallel connected thyristors, where the load typically needs to be high enough to maintain the current in the TRIAC above the holding current.
While state-of-art dimmers are designed for and work well with conventional lighting means, such as incandescent and halogen lamps, a problem arises with the fact that LED lamps only consume approximately ⅕ of the power of these conventional lamps to generate the similar light flux. The largely reduced power, although beneficial to conserving energy, results in different problems in the different types of dimmers, such as visible flicker of the output light, in particular when operating multiple LED lamps connected to a single dimmer. In addition in a leading-edge dimmer, the under-load may pull the current in the TRIAC below the holding current. This causes the TRIAC to be set to a non-conductive or “disconnected” state, also referred to as “early disconnect”.
The object of the present invention therefore is to provide a circuit arrangement for enhancing the operation of a LED lamp when connected with a phase-cut power supply and in particular when multiple lamps are connected to the same phase-cut power supply, so that a versatile use of the LED lamp is possible independent of the configuration of the lighting system.